The analysis of fingerprints, for example, has become the standard method for the positive identification of individuals because of the immense variety of epidermal ridge patterns which, to date, have not been found to repeat themselves, and also because of the invariance of the individual patterns. They do not change with age; the ridges will evolve again as they were before even after an abrasion of the fingertips' epidermis.
The methods of fingerprint pattern analysis are used principally in criminal investigations but also in medicine, genetics, and anthropology, in much the same way as they were introduced by Francis Galton and Edward Henry before the turn of the century. The possibility to analyse and compare fingerprints automatically has been the subject of intense study for the past 25 years with the objectives, firstly, to devise an instrument that could cope with the ever increasing volume of information in police work, and secondly, to reduce the element of bias associated with a subjective interpretation. Lately, it has been realized that the individuality of fingerprints could be the basis for access control systems if an instrument or apparatus were designed that could give a go/no go decision automatically, within a very brief interval after an access request by presentation of a fingerprint, and necessarily within very strict confidence limits of performance.
Fingerprints are classified by assigning a letter or number code to each characteristic element or pattern observed (e.g., `T` for the tented arch, `R` for the radial loop, `W` for the whorl, etc.). The topological constraints on continuous line patterns containing loops or whorls require that triradii exist, i.e., star-like forms where a ridge branches into two. An important element in establishing the fingerprint code is the number of ridges between a triradius and the center of the whorl or loop. The final identification formula for an individual's set of fingerprints is given as a sequence of the code letters and numbers for all ten fingers.
The conventional system of fingerprint analysis is slow and expensive since it requires highly trained personnel. As a result, analyses are performed in specialized centers only, and the consequence is an inevitable delay in the decisions rendered on the basis of fingerprint comparisons. Still more time-consuming is a search to find a fingerprint on file to match the imprint of a single finger. However, situations in which a presumed criminal has to be identified on the basis of a single print arise very frequently.
The relatively slow process of compiling and emitting data has spurred the development of computers that could, (a), recognize fingerprints on the basis of a comparison with a register of patterns stored in a memory bank, (b), measure them with a high degree of precision, (c), produce the conventional code, and (d), search their files for a matching print. Computers performing these tasks are available; opto-electric scanners look first for the location of the principal or determinant pattern in a print and, once found, proceed by measuring methodically the line slopes in these patterns as well as other features necessary to establish the code. Another class of computers uses a direct optical comparison and correlation. Neither of the systems has proved reliable in daily use. A certain data reduction in either approach is inevitable and might have been the cause for the unacceptably high `false reject` as also `false accept` rates. Some companies that have produced computer analysis systems for fingerprints have now ceased the production of these specialized and highly complicated machines.
The difficulties encountered with automated pattern analysis systems for fingerprints probably derive from the fact that the approach adopted tries to simulate too closely the integrating operations of the human eye and brain.
In U.S. Pat. No. 4,025,898 issued May 24, 1977 to Shaw, there is described a method for recording representations of disrupted space patterns, such as fingerprints, so that their classification, comparison and retrieving are simplified. The method makes use of the phenomenon of moire patterns which consists in the superposition or interference of two periodic structures resulting in an optical image having frequency phase differences. Since fingerprints are a periodic line pattern, another periodic structure is then only required to create the moire pattern. Hence, in the above patent, moire interference patterns are created by superimposing a line pattern on a fingerprint, or a print on its own image. When the original and the reference patterns are rotated with respect to each other, the changing interferometric pattern is recorded by a camera: the sequence of lines passing by the camera as well as the variation of amplitude with time of the light signal reflected from, or transmitted through the two superposed patterns expresses the characteristics of the fingerprint. The variation of the moire pattern is recorded.